One or more embodiments of the present invention relate generally to methods for fabricating patterned features utilizing imprint lithography.
There is currently a strong trend, for example and without limitation, in the semiconductor manufacturing industry, toward micro-fabrication, i.e., fabricating small structures and downsizing existing structures. For example, micro-fabrication typically involves fabricating structures having features on the order of micro-meters or smaller.
One area in which micro-fabrication has had a sizeable impact is in microelectronics. In particular, downsizing microelectronic structures has generally enabled such microelectronic structures to be less expensive, have higher performance, exhibit reduced power consumption, and contain more components for a given dimension relative to conventional electronic devices. Although micro-fabrication has been utilized widely in the electronics industry, it has also been utilized in other applications such as biotechnology, optics, mechanical systems, sensing devices, and reactors.
As is well known, methods for fabricating patterned features are an important part of micro-fabrication. In the art of micro-fabrication of, for example and without limitation, semiconductor devices, “lift-off” is a well known method for fabricating patterned metal features such as, for example and without limitation, lines on a substrate or wafer. FIGS. 1A–1D illustrate a well known process for fabricating patterned metal features in which a photoresist mask is undercut by a developer prior to metal deposition. As shown in FIG. 1A, substrate 100 has been coated with photoresist layer 110 in accordance with any one of a number of methods that are well known to those of ordinary skill in the art, and photoresist mask layer 110 has been patterned in accordance with any one of a number of methods that are well known to those of ordinary skill in the art to provide aperture 120 having relatively straight side walls. For example, in accordance with one such lithography technique, photoresist 110 was exposed to a beam of electrons, photons, or ions by either passing a flood beam through a mask or scanning a focused beam. The beam changed the chemical structure of an exposed area of photoresist layer 110 so that, when immersed in a developer, either the exposed area or an unexposed area of photoresist layer 110 (depending on the type of photoresist used) was removed to recreate a pattern, or its obverse, of the mask or the scanning. Next, as shown in FIG. 1B, aperture 120 has been undercut in accordance with any one of a number of methods that are well known to those of ordinary skill in the art to form aperture 130 in photoresist mask layer 110. Next, as shown in FIG. 1C, a relatively thin metal layer has been blanket-deposited over the structure shown in FIG. 1B. As is well known, metal thin film deposition techniques such as, for example and without limitation, physical vapor deposition (“PVD”) or sputtering (and excepting conformal deposition techniques such as, for example and without limitation, chemical vapor deposition (“CVD”) and electroplating) provide limited step coverage. As a result, metal deposited using such techniques does not coat steep or undercut steps. Thus, as shown in FIG. 1C, after blank metal deposition, the undercut side walls of aperture 130 are not coated. In other words, the use of undercut aperture 130 in photoresist mask layer 110 avoids side wall metal deposition, and provides discontinuous metal regions on substrate 100 and photoresist mask layer 110. Lastly, as shown in FIG. 1D, a photoresist lift-off process has been carried out in accordance with any one of a number of methods that are well known to those of ordinary skill in the art to provide patterned metal feature 150 on substrate 100. As is well known, during the lift-off process, photoresist material under metal film 140 is removed using, for example and without limitation, a solvent or a photoresist stripper. As a result, metal film 140 is removed, and patterned metal feature 150 that was deposited directly on substrate 100 remains.
Lithography is an important technique or process in micro-fabrication that is used to fabricate semiconductor integrated electrical circuits, integrated optical, magnetic, mechanical circuits and microdevices, and the like. As is well known, and as was discussed above, lithography may be used to create a pattern in a thin film carried on a substrate or wafer so that, in subsequent processing steps, the pattern can be replicated in the substrate or in another material that is deposited on the substrate. An imprint lithography technology for producing nanostructures with 10 nm feature sizes has been discussed in the literature. One embodiment of imprint lithography—referred to in the art as Step and Flash Imprint Lithography (“SFIL”)—is disclosed in an article by B. J. Smith, N. A. Stacey, J. P. Donnelly, D. M. Onsongo, T. C. Bailey, C. J. Mackay, D. J. Resnick, W. J. Dauksher, D. Mancini, K. J. Nordquist, S. V. Sreenivasan, S. K. Banerjee, J. G. Ekerdt, and C. G. Willson entitled “Employing Step and Flash Imprint Lithography for Gate Level Patterning of a MOSFET Device” SPIE Microlithography Conference, February 2003, which article is available on the Internet at www.molecularimprints.com, and which article is incorporated by reference herein. SFIL is a lithography technique that enables patterning of sub-100 nm features at a cost that has the potential to be substantially lower than either conventional projection lithography or proposed next generation lithography techniques. As described in the article, SFIL is a molding process that transfers the topography of a rigid transparent template using a low-viscosity, UV-curable organosilicon solution at room temperature with low pressure mechanical processes.
One such SFIL process is illustrated in conjunction with FIGS. 2A–2F. As shown in FIG. 2A, thinorganic layer 210 (referred to as a transfer layer) has been spin-coated on silicon substrate 200. Next, a small amount of low viscosity, photopolymerizable, organosilicon solution 220 is dispensed over transfer layer 210 in an area to be imprinted (solution 220 is sometimes referred to as an “imprinting material”). The viscosity of solution 220 is sufficiently low so that minimal pressure (for example and without limitation, a pressure of about 2–4 psi) and no additional heating is necessary to move the liquid into an imprint template. For example, solution 220 may be a solution of an organic monomer, a silylated monomer, and a dimethyl siloxane oligomer (“DMS”) and a multifunctional cross-linker. Each component plays a role in the imaging process. For example: (a) the free radical generator initiates polymerization upon exposure to actinic (typically UV) radiation; (b) the organic monomer ensures adequate solubility of the free radical generator, desirable cohesive strength of cured imprinting material and adhesion to underlying organic transfer layer 210; (c) and the silylated monomers and the DMS provide silicon required to provide high-oxygen etch resistance (useful in subsequent processing steps described below); and (d) multi-functional crosslinker provides chemical crosslinking. In addition, these monomer types help maintain a low viscosity that is useful during imprinting. In further addition, the silylated monomer and the DMS derivative also lower the surface energy of solution 220, thereby enhancing a separation process (described below). Advantageously, the organic monomer polymerizes in a fraction of a second using low cost, broadband light sources. For example, as described in the article, solution 220 consisted of 15% (w/w) ethylene glycol diacrylate (obtained from Aldrich Chemical Company of Milwaukee, Wis.), 44% (3-acryloxypropyl)tris(trimethylsiloxy)silane (obtained under the trade name SIA0210.0 from Gelest, Inc. of Morrisville, Pa.), 37% t-butyl acrylate (obtained from Lancaster Synthesis Inc. of Windham, N.H.), and 4% 2-hydrozy-2-methyl-1-phenyl-propan-1-one (obtained under the trade name Darocur 1173 from CIBA® of Tarrytown, N.Y.).
Next, as shown in FIG. 2B, template 230—bearing patterned relief structures (for example and without limitation, a circuit pattern) and whose surface was treated with a fluorocarbon release film—was aligned over dispensed solution 220 and moved to decrease a gap between template 230 and substrate 200. This displaced solution 220, and filled the patterned relief structures on template 230. Suitable release layers are described in an article by D. J. Resnick, D. P. Mancini, S. V. Sreenivasan, and C. G. Willson entitled “Release Layers for Contact and Imprint Lithography” Semiconductor International, June 2002, pp. 71–80, which article is incorporated by reference herein. As is known, it is desired that a template release layer has a low enough surface energy to enable template/substrate separation, and also is reasonably durably bonded to the template surface to remain functional after a number of imprints. Alkyltrichlorosilanes form strong covalent bonds with a surface of fused silica, or SiO2. In addition, in the presence of surface water, they react to form silanol intermediates which undergo a condensation reaction with surface hydroxyl groups and adjacent silanols to form a networked siloxane monolayer. When this functional group is synthetically attached to a long fluorinated aliphatic chain, a bifunctional molecule suitable as a template release film may be created. The silane-terminated end bonds itself to a template's surface, providing durability useful for repeated imprints. The fluorinated chain, with its tendency to orient itself away from the surface, forms a tightly packed comb-like structure, and provides a low-energy release surface. Annealing further enhances the condensation, thereby creating a highly networked, durable, low surface energy coating.
Next, as shown in FIG. 2C, once filling has occurred, the area is irradiated with broadband UV ultraviolet light (for example and without limitation, a 500 W Hg arc lamp) through a back side of template 230, and cross-linking of solution 220 occurs.
Next, as shown in FIG. 2D, template 230 and substrate 200 are mechanically separated to expose cured, organosilicon relief pattern 240 (an imprinted version of the relief pattern in template 230) that is disposed on residual layer 250 (a residue of cross-linked solution 220). The SFIL steps illustrated in FIGS. 2A–2D may be carried out in a tool described by I. McMackin, P. Schumaker, D. Babbs, J. Choi, W. Collison, S. V. Sreenivasan, N. Schumaker, M. Watts, and R. Voisin in an article entitled “Design and Performance of a Step and Repeat Imprinting Machine” SPIE Microlithography Conference, February 2003, which article is available on the Internet at www.molecularimprints.com, and which article is incorporated by reference herein.
Next, etching is performed in a two-step process. S. C. Johnson, T. C. Bailey, M. D. Dickey, B. J. Smith, E. K. Kim, A. T. Jamieson, N. A. Stacey, J. G. Ekerdt, and C. G. Willson describe suitable etch processes in an article entitled “Advances in Step and Flash Imprint Lithography” SPIE Microlithography Conference, February 2003, which article is available on the Internet at www.molecularimprints.com, and which article is incorporated by reference herein. As set forth in the article, the first etch step, referred to as a “break-through etch,” anisotropically removes residual cross-linked layer 250 to break through to underlying transfer later 210. The second etch step, referred to as a “transfer etch,” uses the remaining cross-linked relief pattern 240 as an etch mask to transfer the pattern into underlying transfer layer 210. In one embodiment of SFIL, silicon in polymerized solution 220, and lack of silicon in transfer layer 210, provides etch selectivity between polymerized solution 220 and transfer layer 210. In such an embodiment, the etching may be done in a LAM Research 9400SE obtained from Lam Research, Inc. of Fremont, Calif.
As shown in FIG. 2E, a halogen “breakthrough etch” was performed. For example and without limitation, the halogen etch described in the article was an anisotropic halogen reactive ion etch (“RIE”) rich in fluorine, i.e., wherein at least one of the precursors was a fluorine-containing material (for example and without limitation a combination of CHF3 and O2, where the organosilicon nature of solution 220 called for the use of a halogen gas). Other suitable halogen compounds include, for example and without limitation, CF4. This etch is similar to a standard SiO2 etch performed in modern integrated circuit processing. Lastly, as shown in FIG. 2F, an anisotropic oxygen reactive ion etch was used to transfer features 260 to underlying substrate 200. The remaining silicon containing features 260 served as an etch mask to transfer the pattern to underlying substrate 200. The “transfer etch” was achieved with a standard, anisotropic, oxygen RIE processing tool.
In order to imprint sub-100 nm features, it is useful to avoid intermixing between an imprinting material and a transfer layer. Intermixing may cause problems such as, for example and without limitation, distortion of features when an imprint template is separated from a substrate after exposure to polymerizing radiation. This can be particularly problematic when feature thicknesses are as small as 50 to 100 nm. In addition, intermixing may be particularly problematic when using an imprinting material comprised of low viscosity acrylate components because such components have solvency toward many polymers. Because of this, some have used a cross-linked BARC material (BARC or “bottom antireflective coating” is an organic antireflective coating that is typically produced by a spin-on process) as a transfer layer. However, because BARC is cross-linked, it cannot be undercut by conventional wet developers and removed by organic photostrippers. As a result, the above described method for fabricating patterned metal features using lift-off cannot be used.
In light of the above, there is a need for methods for fabricating patterned features utilizing imprint lithography that overcome one or more of the above-identified problems.